Cloning and Characterization of a Chick Embryo CDNA and Gene for IGF-binding Protein-2

We have isolated and characterized a cDNA for IGF-binding protein-2 (IGFBP-2) and its gene from the chick embryo. Using primers from a conserved region of the mammalian IGFBP-2 sequence, a cDNA clone (1.6 kb) was isolated from an embryonic day-18 chick retina cDNA library. Although the clone was truncated at the 5' end, the complete coding sequence was obtained from 5' rapid amplification of cDNA ends and genomic sequencing. The open reading frame encoded a 311 amino acid precursor protein which contains a putative 36 residue signal peptide. The mature 275 amino acid protein had a predicted M(r) of 33,500 and exhibited 71, 68, 68 and 66% identity to rat, bovine, ovine and human IGFBP-2 cDNA respectively, with conservation of all 18 cysteines. The cDNA contained an RGD peptide but lacked a putative ATP-binding motif. A single transcript of approximately 2.3 kb was present in embryonic day-15 eye, brain, skeletal muscle, heart and intestine, but was virtually absent from embryonic day-15 liver. The chicken IGFBP-2 gene spanned approximately 38 kb, consisted of four exons, and was similarly organized to that of the rat and human. Southern blot analysis of chicken genomic DNA suggested that it is encoded by a single gene. The sequence information from the avian IGFBP-2 should be of value in examining the role of IGFBP-2 in vertebrate development.